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151.2.3 Multiple Lines of Evidence for Climate Change
While the first IPCC assessment depended primarily on observed changes in surface temperature and climate model analyses, more recent assessments include multiple lines of evidence for climate change. The first line of evidence in assessing climate change is based on careful analysis of observational records of the atmosphere, land, ocean and cryosphere systems (Figure 1.3). There is incontrovertible evidence from in situ observations and ice core records that the atmospheric concentrations of GHGs such as CO₂, CH4, and N₂O have increased substantially over the last 200 years (Sections 6.3 and 8.3). In addition, instrumental observations show that land and sea surface temperatures have increased over the last 100 years (Chapter 2). Satellites allow a much broader spatial distribution of measurements, especially over the last 30 years. For the upper ocean temperature the observations indicate that the temperature has increased since at least 1950 (Willis et al., 2010;Willis, J., D. Chambers, C. Kuo, and C. Shum, 2010: Global sea level rise recent progress and challenges for the decade to come. Oceanography, 23, 26–35. Section 3.2). Observations from satellites and in situ measurements suggest reductions in glaciers, Arctic sea ice and ice sheets (Sections 4.2, 4.3 and 4.4). In addition, analyses based on measurements of the radiative budget and ocean heat content suggest a small imbalance (Section 2.3). These observations, all published in peer-reviewed journals, made by diverse measurement groups in multiple countries using different technologies, investigating various climate-relevant types of data, uncertainties and processes, offer a wide range of evidence on the broad extent of the changing climate throughout our planet. Conceptual and numerical models of the Earth’s climate system offer another line of evidence on climate change (discussions in Chapters 5 and 9 provide relevant analyses of this evidence from paleoclimatic to recent periods). These use our basic understanding of the climate system to provide self-consistent methodologies for calculating impacts of processes and changes. Numerical models include the current knowledge about the laws of physics, chemistry and biology, as well as hypotheses about how complicated processes such as cloud formation can occur. Because these models can represent only the existing state of knowledge and technology, they are not perfect; they are, however, important tools for analysing uncertainties or unknowns, for testing different hypotheses for causation relative to observations, and for making projections of possible future changes. One of the most powerful methods for assessing changes occurring in climate involves the use of statistical tools to test the analyses from models relative to observations. This methodology is generally called detection and attribution in the climate change community (Section 10.2). For example, climate models indicate that the temperature response to GHG increases is expected to be different than the effects from aerosols or from solar variability. Radiosonde measurements and satellite retrievals of atmospheric temperature show increases in tropospheric temperature and decreases in stratospheric temperatures, consistent with the increases in GHG effects found in climate model simulations (e.g., increases in CO₂, changes in O3), but if the Sun was the main driver of current climate change, stratospheric and tropospheric temperatures would respond with the same sign (Hegerl et al., 2007).Hegerl, G. C., et al., 2007: Understanding and attributing climate change. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change S., D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor and H. L. Miller (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 665–745. Resources available prior to the instrumental period—historical sources, natural archives, and proxies for key climate variables (e.g., tree rings, marine sediment cores, ice cores)—can provide quantitative information on past regional to global climate and atmospheric composition variability and these data contribute another line of evidence. Reconstructions of key climate variables based on these data sets have provided important information on the responses of the Earth system to a variety of external forcings and its internal variability over a wide range of timescales (Hansen et al., 2006Hansen, J., M. Sato, R. Ruedy, K. Lo, D. W. Lea, and M. Medina-Elizade, 2006: Global temperature change. Proc. Natl. Acad. Sci. U.S.A., 103, 14288–14293.; Mann et al., 2008)Mann, M., Z. Zhang, M. Hughes, R. Bradley, S. Miller, S. Rutherford, and F. Ni, 2008:Proxy-based reconstructions of hemispheric and global surface temperature variations over the past two millennia. Proc. Natl. Acad. Sci. U.S.A., 105, 13252– 13257.. Paleoclimatic reconstructions thus offer a means for placing the current changes in climate in the perspective of natural climate variability (Section 5.1). AR5 includes new information on external RFs caused by variations in volcanic and solar activity (e.g., Steinhilber et al., 2009;Steinhilber, F., J. Beer, and C. Fröhlich, 2009: Total solar irradiance during the Holocene. Geophys. Res. Lett., 36, L19704. see Section 8.4). Extended data sets on past changes in atmospheric concentrations and distributions of atmospheric GHG concentrations (e.g., Lüthi et al., 2008;Lüthi, D., et al., 2008: High-resolution carbon dioxide concentration record 650,000– 800,000 years before present. Nature, 453, 379–382. Beerling and Royer, 2011)Beerling, D. J., and D. L. Royer, 2011: Convergent Cenozoic CO2 history. Nature Geosci., 4, 418–420. and mineral aerosols (Lambert et al., 2008)Lambert, F., et al., 2008: Dust-climate couplings over the past 800,000 years from the EPICA Dome C ice core. Nature, 452, 616–619. have also been used to attribute reconstructed paleoclimate temperatures to past variations in external forcings (Section 5.2). Notes Navigation ES 1.1 1.2.1 1.2.2 1.2.3 1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.3.4.1 1.3.4.2 1.3.4.3 1.4.1 1.4.2 1.4.3 1.4.4 1.5 1.5.1 1.5.2 1.6 Box 1 FAQ Refs